Recently, interest in energy storage technologies is increasing day by day. As the energy storage technologies are extensively applied to mobile phones, camcorders and notebook computers, and further to electric vehicles, the demand for high energy densification is increasing in the field of batteries used as a power source of such electronic appliances. Lithium secondary batteries are the most suitable to meet the demand, and thus, their study is being made actively at present.
Among the currently available secondary batteries, lithium secondary batteries developed in the early 1990's comprise an anode of a carbon material capable of intercalating and disintercalating lithium ions, a cathode of lithium containing oxide and a non-aqueous electrolyte solution having a proper amount of lithium salt dissolved in a mixed organic solvent.
The lithium secondary batteries generally have an average discharge voltage between about 3.6V to about 3.7V. The lithium secondary batteries advantageously have a higher discharge voltage than alkali batteries, nickel-cadmium batteries and so on. To exhibit such a high operating voltage, it needs an electrolyte solution composition which is electrochemically stable in a charge/discharge voltage range between 0 to 4.2V. For this purpose, an electrolyte solution uses, as a solvent, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate, propylene carbonate, etc. is properly mixed with a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, etc. And, the electrolyte solution generally uses, as a solute, a lithium salt including LiPF6, LiBF4, LiClO4 and so on. These lithium salts serve as a source of lithium ions to enable operation of lithium secondary batteries.
At the initial charge of lithium secondary batteries, lithium ions emitted from a cathode active material such as lithium metal oxide transfer to an anode active material such as graphite, and intercalate into layers of the anode active material. At this time, because lithium has strong reactivity, an electrolyte solution reacts with carbon of the anode active material, such as graphite, to produce a compound such as Li2CO3, Li2O, LiOH and so on. These compounds form a so-called solid electrolyte interface (SEI) film on the surface of the anode active material such as graphite.
The SEI film acts as an ion tunnel, and enables only lithium ions to pass therethrough. Such ion tunnel effect of the SEI film prevents structural destruction of an anode that may be resulted from intercalation of molecules of an organic solvent having a high molecular weight into layers of the anode active material while migrating with lithium ions in the electrolyte solution. As a result, it prevents a contact between the electrolyte solution and the anode active material, and consequently reduces decomposition of the electrolyte solution. And, it can reversibly maintain an amount of lithium ions in the electrolyte solution, resulting in stable charge/discharge.
However, a thin-film prismatic battery may swell in thickness during charge due to CO, CO2, CH4, C2H6, etc that is generated by decomposition of a carbonate-based organic solvent occurring when an SEI film is formed as mentioned above. Furthermore, when a battery is left at a high temperature in a fully charged state, the SEI film may be slowly broken down due to increased electrochemical energy and thermal energy over time. It induces continuous side reactions between the exposed surface of an anode and a surrounding electrolyte solution, and consequently gas is continuously generated. The gas increases the inner pressure of the battery. As a result, the battery, particularly for example, a prismatic battery and a pouch-shaped battery have the increased thickness, which may cause performance problems to electronics such as mobile phones, notebook computers and so on. That is, stability at high temperature is lowered. Because a typical lithium secondary battery contains a great amount of ethylene carbonate, the lithium secondary battery is more vulnerable to the above-mentioned inner pressure increase problem caused by an unstable SEI film. In order to solve these problems, suggestion has been made to add an additive to a carbonate-based organic solvent so as to change the aspect of reactions occurring when an SEI film is formed. However, when a certain compound is added to an electrolyte solution to improve the battery performance, the characteristics of the battery are improved in some aspects, but may be deteriorated in some aspects.